99% of Developers Get This SQL Question Wrong in Interviews

I’ve been Using SQL UNION Wrong for 3 Years — Here’s What I Learned

The Question That Breaks Confident Developers

“What’s the difference between UNION and UNION ALL?”

It seems simple enough. Most developers confidently start explaining: “UNION removes duplicates, UNION ALL doesn’t.” Then the interviewer asks the follow-up that changes everything:

“When would you use each one?”

That’s when the stumbling begins.

I’ve conducted over 200 technical interviews in the past five years, and this exact scenario plays out repeatedly. Developers who can write complex joins and optimize query plans suddenly go quiet when asked to choose between UNION and UNION ALL in a real-world context.

Here’s what actually happens:

The candidate usually picks UNION “to be safe” — after all, who wants duplicates, right? But that single choice reveals a fundamental misunderstanding that could cost their team thousands in database performance costs.

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The Real Problem

It’s not that developers don’t know the technical difference. The issue is they’ve never been taught when and why to use each operator. They memorize the syntax but miss the strategy.

In production systems, choosing UNION over UNION ALL without thinking can:

• Slow down queries by 300–500%

• Consume unnecessary memory resources

• Create bottlenecks that scale poorly

• Signal to interviewers that you don’t understand performance implications